1. Field of the Invention
This invention relates to optical disc drives, and more particularly, to a method of automatically detecting the current position of the pickup head of the optical disc drive to thereby switch the motor operation of the optical disc drive between the CLV (constant linear velocity) mode and the CAV (constant angular velocity) mode without interrupting the reading and transferring of data from the optical disc.
2. Description of Related Art
The optical disc is a mass storage medium that can hold up to several megabits or even gigabits of binary data in a single piece of plastic disc, which is several times larger than the capacity of a conventional magnetic disk. Due to its high capacity, the optical disc is now widely used in the computer arena as the major data storage medium.
The optical disc is typically formed with a spiral-shaped track on which data are stored, in a radially outward manner from the innermost radial position to the outermost radial position. Therefore, the amount of data stored at a particular radial position is proportional to the distance between that position and the center of the optical disc. During read operation, the optical disc is rotated by a spindle motor, which can be controlled in either of the following two modes: constant linear velocity (CLV) mode and constant angular velocity (CAV) mode.
When the spindle motor is operating in the CLV mode, the speed of the spindle motor decreases linearly as the pickup head is moved farther from the center of the optical disc, as illustrated in FIG. 1A, if maintenance of a fixed data transfer rate is desired, as illustrated in FIG. 1B. It can be seen that if a higher data transfer rate is desired, a higher speed spindle motor should be utilized, which can cause the implementation cost to be high. Moreover, when the spindle motor is operating in the CLV mode, the spindle motor is subjected to frequent changes in speeds, which can cause the spindle motor to consume more electrical power and also cause an increase in the access time. These drawbacks are particularly serious if the optical disc drive is doubled in speed.
On the other hand, when the spindle motor is operating in the CAV mode, the data transfer rate increases linearly as the pickup head is moved farther from the center of the optical disc, as illustrated in FIG. 2A, if the speed of the spindle motor is maintained at a fixed value as illustrated in FIG. 2B. The CAV mode of operation is a solution to the above-mentioned problems of the CLV mode of operation, in that it allows the spindle motor to always rotate at a fixed speed irrespective of the current radial position of the pickup head. Therefore, the CAV mode of operation has the benefits of allowing a consistent seek time that is not varied due to any changes in the speed of the spindle motor, and also of reducing power consumption to below that of the CLV mode. It is to be noted that the CAV mode of operation herein is different from the CAV mode of operation for the spindle motor in hard disk drive, since the optical disc uses a spiral-shape track for data storage while the hard disk uses a number of concentric tracks for data storage.
It is a drawback of the CAV mode of the optical disc drive that the data density varies with the radial position of the pickup head. When the pickup head is located at the outermost radial position, it causes the highest data transfer rate, i.e., the highest output radio frequency (RF) from the pickup head; when located at the innermost radial position, it causes the lowest data transfer, i.e., the lowest RF from the pickup head. The inconsistency in the RF can be solved by providing an increase in the data phase-locked loop (PLL) bandwidth.
FIG. 3 is a schematic block diagram of a CLV control circuit for controlling the spindle motor 11 of an optical disc drive. The spindle motor 11 operates in the CLV mode so as to rotate an optical disc 10 during the read operation. In this control circuit, the feedback signal is not obtained from a speed encoder, as it is in most conventional control circuits, but instead is extracted from the EFM signal obtained from the optical disc 10. As mentioned earlier, the purpose of the CLV mode is to maintain the data transfer rate at a fixed value. To achieve this, the fundamental frequency data Pck4m extracted from the EFM signal is compared with a reference signal Vck4m, and the resulting error signal is used in the feedback control for the speed of the spindle motor 11. As mentioned earlier, the amount of data stored at a particular radial position is proportional to the distance between that radial position and the center of the optical disc. Therefore, in order to maintain the data transfer rate at a fixed value, the motor speed is increased when the pickup head is moved radially outwards, and decreased when the pickup head is moved radially inwards. This relationship is shown in FIGS. 1A-1B.
FIG. 4 is a schematic block diagram of a CAV control circuit for controlling the spindle motor 21 of an optical disc drive, so that the spindle motor 21 operates in the CAV mode. In this control circuit, the feedback signal indicative of the current speed of the spindle motor 21 is obtained from the output signal FG of a Hall device 20 which is coupled to the spindle motor 21. As mentioned earlier, the purpose of the CAV mode is to maintain the angular speed of the spindle motor 21 at a fixed value. To achieve this, the FG signal from the Hall device 20 is compared with a reference signal Vck4m to thereby obtain an error signal that is used in the feedback control for the speed of the spindle motor 21. As mentioned earlier, the amount of data stored at a particular radial position is proportional to the distance between that radial position and the center of the optical disc. Therefore, with the motor speed being fixed at a constant value, the data transfer rate will be increased when the pickup head is moved radially outwards, and decreased when the pickup head is moved radially inwards. This relationship is shown in FIGS. 2A-2B.
As a summary, the foregoing two conventional motor control methods have the following disadvantages.
(1) First, in the CLV mode of operation, the inconsistency in the speed of the spindle motor when the pickup head is moved to different radial positions causes the seek time to be lengthy and the power consumption of the spindle motor to be high. Moreover, the data transfer rate will be limited to a low rate. PA1 (2) Second, although partitioned read operations in the CLV mode can increase the data transfer rate, the drawbacks of a lengthy seek time and high power consumption are still unsatisfactory when using this scheme. PA1 (3) Third, in the CAV mode of operation, although the spindle motor is low in power consumption and quick in seek time due to the motor speed being always fixed at a constant value, it nonetheless has an undesirably low average data transfer rate since the data transfer rate is very low when the pickup head is reading data from the innermost radial position of the optical disc.